The answer is: Yes if you have an accident that involves your head hitting something. No if you have an accident in which your head hits nothing but any other part of your body hits something.

Helmets, I wear one sometimes, not at other times, it's a whim according to whether I think my environment (rush hour London) or cycling (racing or fast downhills) might warrant a helmet, and not when I think my leisurely pace reduces risk.

They aren't a lifesaver if you're going to be hit by a truck, the false sense of security they give new cyclists is probably the biggest problem with them... oh, and they never look good, on anyone, period.